One known type of information storage device is a disk drive device that uses magnetic media to store data and a movable read/write head that is positioned over the magnetic media to selectively read from or write to the magnetic media.
A conventional hard disk drive (HDD) generally includes a magnetic hard disk, a spindle motor for rotating the disk at a high speed, a head stacking assembly (HSA) with a slider configured to read data from and write data to a magnetic hard disk, a voice coil motor (VCM) provided for controlling the motion of the head stacking assembly (HSA), all of which are mounted into a housing.
Nowadays, two types of voice coil motor are manufactured and sold in the market. One popular type of voice coil motor (VCM) incorporated in a conventional hard disk drive includes an actuator coil and two magnets which are separately located on the bottom and top of the actuator coil, such a voice coil motor is called “dual magnets design”. This type of voice coil motor has dual magnets to form a close magnetic filed, thereby having less magnetic flux loss and achieving bigger magnetic flux density. Normally, the bigger magnetic flux density will generate the bigger torque force and a better linearity of torque force is of benefit to improve the reading and writing performance of HDD.
As shown in FIG. 1, another type of voice coil motor 800 includes only one magnet 810 which is located on the bottom or top of the actuator coil 820, it is called “single magnet design”. Due to single magnet structure, the VCM 800 with single magnet design can reduce the manufacture and assembly cost of the VCM. Furthermore, the single magnet design has less surface area, so it can save the raw material including nickel which is plated on the surface of magnet 810, thereby reducing material cost and environment pollution. However, the magnet 810 has a neutral zone 813 which is the transition surface of magnetic north polar 811 and magnetic south polar 812, the magnetic flux density of neutral zone 813 is lower than other area of the magnet 810, whereas the existing single magnet 810 can not form a close magnetic filed to reduce magnetic flux loss, thus, the single magnet 810 has smaller magnetic flux density than dual magnets on the neutral zone area. So the torque force produced by the conventional single magnet 810 is lower than that produced by dual magnets on the close neutral zone area. Therefore, in order to make full use of the neutral zone area in the single magnet 810, we need to increase the torque force in the neutral zone area to improve the linearity of torque force from inner diameter (ID) to outer diameter (OD) of HDD.
Accordingly, it is desired to provide a voice coil motor with new magnet design to improve the linearity of torque force and then overcome the above-mentioned drawbacks.